Processes for hydroformylating an olefin to prepare a carbonyl derivative containing one carbon atom more than the parent olefin by reacting the olefin with synthesis gas in the presence of a Group VIII metal, e.g. rhodium, in complex combination with an organic ligand, carbon monoxide also being a component of the catalyst complex, are well known in the art and of growing industrial importance. This technology is summarized, for example, in U.S. Pat. No. 3,527,809 to Pruett et al. The olefin reactant is contacted with the catalyst and the synthesis gas (a mixture of carbon monoxide and hydrogen) in the presence of a liquid reaction medium, which may or may not comprise a separate inert liquid solvent species. A gas comprising the carbon monoxide and hydrogen is typically bubbled through the liquid reaction medium which is contained in a hydroformylation reactor which can be mechanically stirred or which may be agitated solely by the action of reactant gas being bubbled therethrough. The gas, in addition to hydrogen and carbon monoxide, may also contain vapors of the reactant olefin, in a proportion which will depend upon such factors as reaction conversion rate and the volatility of the olefin.
The aldehyde hydroformylation product can be recovered from the liquid hydroformylation reaction medium in various ways, but, especially when the aldehyde is of comparatively low molecular weight, e.g., when it contains from three to about seven carbon atoms and especially when it contains from three to about five carbon atoms, it is conveniently stripped out in vapor form by distillation, evaporation, or, especially, by being stripped out of the hydroformylation reaction zone in the gases which are being bubbled through the liquid contained therein. Hershman et al have described this technology in "I&EC Product Research and Development" 8, pp 372-375 (1969) in a discussion of the hydroformylation of propylene in a gas-sparged reactor.
In more recent years various patents and other publications have appeared directed to the use of special reaction solvents and/or special techniques for stripping the aldehyde product out of the liquid reaction medium. For example, U.S. Pat. No. 4,148,830 (Pruett et al) recommends using high-boiling reaction by-products as the reaction solvent, with the aldehyde product being subsequently recovered from the reaction medium in a separate vaporization operation.
In a related hydroformylation process, U.S. Pat. No. 4,329,511 issued to Hackman et al describes the use of a high molecular weight, high-boiling inert liquid reaction solvent in proportions of about 40 to about 95% by weight of the liquid reaction product, for purposes of controlling the rate of stripping at a level such that at a given molar concentration (relatively low) of product aldehyde in the mixture, and the mole fraction of the aldehyde in the mixture will be relatively high. The solvents disclosed as suitable are those having a molecular weight of at least about 700 and are capable of dissolving the catalyst and olefin. The specific solvents used are not considered critical as long as they are: (1) miscible with the catalyst system; (2) miscible with the reactants; (3) miscible with the reaction products; (4) low in volatility so as to facilitate stripping reaction product and by-products from it and (5) chemically inert in the hydroformylation reaction system. The disclosed solvents include, for example, alkyl-substituted benzene; pyridine and alkyl-substituted pyridines; tertiary amines; high boiling esters such as dialkyldicarboxylates and triorganophosphates as well as esters of polyols such as trimethylolpropane and pentaerythritol; ketones; alcohols such as butanols; nitriles such as acetonitriles; and hydrocarbons such as kerosene, mineral oil, cyclohexane, naphtha, etc. and aromatics such as biphenyl. In addition to these solvents, the use of polyalkylene glycols such as polyethylene glycol and polypropylene glycol having molecular weights greater than about 700 are stated to be particularly desirable because of their availability and their desirable properties for use as a hydroformylation solvent.
A further related hydroformylation process is described in U.S. Pat. No. 4,151,209 to Paul et al, which describes techniques for recovering aldehyde products from the reaction products by distillation, stripping, employing the ratio of phosphorus contained in the high-boiling reaction by-products to the phosphorus contained in the ligand (triorganophosphine ligand) which is present, as the primary control. Although the claimed improvement of the Paul et al process is different from that of the process of U.S. Pat. No. 4,329,511, similar solvents have been found to be satisfactory for use in the hydroformylation reactions described in both of these patents.
As has been described above, there are many satisfactory solvents which can be used in the hydroformylation of olefins to aldehydes. It is the objective of this invention to provide a hydroformylation process using a new solvet which has all the necessary properties required of a hydroformylation solvent but also provides improvements in the efficiency of the production and recovery of aldehydes from the hydroformylation reaction products.